intel-iommu: Mask physical address to correct page size in intel_map_single()

The physical address passed to domain_pfn_mapping() should be rounded 
down to the start of the MM page, not the VT-d page.

This issue causes kernel panic on PAGE_SIZE>VTD_PAGE_SIZE platforms e.g. ia64
platforms.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>